Line up elements on themes.php. Line up 'Available Themes' header and...

Line up elements on themes.php. Line up 'Available Themes' header and search/feature form. Switch to padding instead of margin for #current-theme to prevent margin-uncollapse jitters when animating the feature filter. Move 'Current Theme' inside #current-theme so it is hidden by the feature filter. see #20147. 


git-svn-id: https://core.svn.wordpress.org/trunk@20064 1a063a9b-81f0-0310-95a4-ce76da25c4cd